In various systems which recirculate an aqueous medium, such as boiler systems for energy production, the recirculated water picks up unwanted oxygen which becomes dissolved in the water. The presence of the dissolved oxygen can be involved in a variety of corrosion problems within the secondary system of a power plant, such as pitting, denting of steam generator tubes, stress corrosion cracking and feedtrain degradation. In the predominant process for dissolved oxygen removal, in a secondary system of a power plant, hydrazine is added at the condensate pump. Even though oxygen ingress can occur in any section of the feedtrain from the condenser to the steam generator, such hydrazine addition is only effective at temperature, conditions which prevail after the feedwater heaters. This is due to the kinetics for the oxygen-hydrazine reaction being more favorable at high temperatures.
Current dissolved-oxygen control practices in power plant systems are limited for various reasons. One reason is the relatively short contact time available for hydrazine to react with oxygen in the high temperature region of the feedtrain where this reaction is most efficient. In the low temperature end of the feedtrain, the region immediately following the condenser and up to the feedwater heaters, little oxygen removal occurs because of low temperatures that prevail at that region. Another complication with the current hydrazine addition processes is that, in plants having copper alloy feedtrain components, there is competition between the copper surfaces and hydrazine for the dissolved oxygen. The reaction of copper and oxygen leads to the formation of nonvolatile surrogates of oxygen which may be transported into the steam generator or can be active in steam generator corrosion processes. Due to the large surface areas of copper alloys in the feedtrain, they can react more readily with oxygen than the available hydrazine due to limited contact time.
Problems involved with the presence of dissolved oxygen in water and various methods for removal of such oxygen, both physical and chemical, are discussed in the paper, "Saving Energy by Catalytic Reduction of Oxygen in Feedwater", by F. Martinola, presented at the 41st Annual Meeting International Water Conference, at Pittsburgh, Pa. on Oct. 20-22, 1980, the contents of said paper being incorporated by reference herein. The Martinola paper discusses the use of a palladium containing polystyrene-based anion exchange resin to aid in the reaction of hydrogen, added to an aqueous medium, with dissolved oxygen in the aqueous medium. In a system described therein, hydrogen is added to water, containing 8 ppm oxygen, which is passed through a mixing tank containing plexiglass rings, and the mixture then flows to a reactor which contains a palladium catalyst. The residual oxygen content of the water after passage through the catalyst column was 0.025 ppm (25 ppb). Other systems using hydrogen and the palladium catalyst gave residual oxygen contents of between about 0.015 ppm to 0.025 ppm (15-25 ppb).
It is an object of the present invention to provide an improved process for removing dissolved oxygen from water using hydrazine, which process can be operated at ambient temmperatures.
It is another object of the present invention to provide a process for removal of dissolved oxygen from an aqueous media that will provide a residual oxygen content of less than 10 parts per billion (ppb) in the aqueous media.
It is a further object of the present invention to provide a process for the removal of dissolved oxygen from the feedtrain of a power plant system before recirculated water is passed through the feedtrain heaters into the steam generator, thereby reducing copper component degradation and transport.